You can kill Siva for a bunch of loot although I don't know what effect that has

You can prevent her dying after you perform the ritual the second time.
At the beginning of the ritual that you perform with your main character (let's say Lohse), Meistr Siva will be sitting on a chair.
Usually after you perform the ritual the second time and you learn a new skill, after this, you'll talk to Meistr Siva with your main character and then some monsters appear and kill Meistr Siva.
You can 't prevent her death by teleporting her across the room, before speaking to her, because the monsters launch projectiles that will travel through walls - the game is made so that she dies then.

To prevent her death, start the ritual the second time (with your main character), go to the Hall of Echoes and learn the new skill, but before returning to the basement of Meistr Siva's house, teleport her with a character that is alone, not chained with other characters from your party and that is not used for bartering with Meistr Siva (because you'll make her mad and you have to pay her afterwards in order to trade with her); and put the rest of your party away from this character that does the teleporting, and away from Meistr Siva, because Meistr Siva will start the combat with your character.
You only want one character to be in a fight with her.

So teleport Meistr Siva away from her chair. Then return with your main character from the Hall of Echoes - you will return near the Meistr Siva's chair and table.
During your return one of your characters is engaged in the fight with Meistr Siva.
Just walk away from the fight by going up to the first floor (you can use invisibility, but it shouldn't be necessary).
If you talk to her with any of your characters after you get the new skill you won't find out anything useful, so Meistr Siva is not useful in the story from now on.

If you talk to her with your main character you'll trigger the appearance of 2 monsters and the death of Meistr Siva.
So don't talk to her from now on with your main character.
If you want to trade with her use a character that is not your main one.

does her request of having a statue in arx actually come true

The way to save Siva is to exit the hall of echoes using your pyramid. Using the cloud to exit is the trigger for the void flayers to spawn. This ofcourse, achieves nothing, as Siva is supposed to die.

If you think you're unable to use your pyramid. Bind scrollwheel up/down to hotbar '1'. Put the pyramid on '1' and spam your scrollwheel. Now you can use the pyramid to exit the HoE (given that you have another pyramid elsewhere on the map)

All the info on how to save her and she didn't die in my run even though I literally didn't even try to save her, none of the monsters spawned for me after second ritual

I thought I'd be smart and try to save Siva by not untying her until after I got to the point where she was supposed to die. Went to release her from the gallows only to find two voidwoken standing there and her already dead.

Use spirit vision after she dies to talk to her. She will ask for a statue in arx near fountain.

i'd give divinity to build that statue